Consequent to the prevalence of the internet in recent years, communication traffic is increasing. Thus, optical fiber transmission technology, in which optical signals are carried by a single optical fiber, is used as a technique for coping with the increases in communication traffic. In addition to the extension of primary trunk systems over long distances and high capacity systems, more flexible and more economical transport networks are being built and phototonic networks are being configured for a society reliant on large volumes of information. These networks are not only metros (metropolitan area networks), but are assumed to be drawn in proximately to offices and residences, and lower cost and more flexible phototonic networks are demanded.
At each node of such systems, an output function of switching paths according to destination is demanded of conventional optical cells or time slots (optical packets). In a communication scheme to build a flexible network, multiple optical packets may be output to the same path and a collision of optical signals may occur. Optical signals that have collided with one another, for example, cannot be read, result in reception errors, and are discarded. The discarding of optical packets not only affects system reliability, but also causes the transmitter to repeatedly send the optical packets until the optical packets are received normally, which decreases the efficiency of the network.
To prevent such circumstances, the scheduling and buffering of the various types of optical signals arbitrarily transmitted from various senders to the nodes is demanded. A scheduler manages the timing at which optical signals are sent, based on the destination of the optical signal. As a buffer, a configuration is known where an optical signal is converted into an electrical signal and stored in an electronic buffer, from which the signal is taken and converted back into an optical signal and sent. Nonetheless, since opto-electronic conversion and electro-optic conversion are used, there are limits in terms of capacity and speed, resulting in increases in power consumption as well as in the complexity and size of the apparatus.
In contrast, optical buffers that delay optical signals as is in the form of light are under investigation (see, for example, Japanese Laid-Open Patent Publication Nos. H8-23306, 2003-57698, 2003-207812, and 2001-242494). An optical buffer prevents the collision of optical signals input at an identical timing and are used not only for decreasing the optical packet discard rate, but also in controlling the transmission sequence of optical signals such that optical signals of high priority are transmitted first. As a conventional optical buffer, a configuration is known in which, for example, multiple optical fibers of differing lengths are arranged in parallel. In this configuration, when an optical signal is to be delayed by a long period, the optical signal is transmitted through a long distance optical fiber and when the optical signal is to be delayed by a short period, the optical signal is passed through a short distance optical fiber, whereby the delay period is adjusted to a desired period.
Nonetheless, with the conventional technologies above, a problem arises in that the size of a configuration to induce a given delay on an optical signal becomes large. For example, in the configuration where multiple optical fibers of differing lengths are arranged in parallel, multiple optical fibers of several hundred [m] to several [km] have to be preliminarily installed, whereby the scale of the hardware becomes large.
For example, in a conventional method such as that of preparing fibers of differing lengths, the configuration of the large-scale hardware is complicated and the introduction of the actual system is not only unrealistic, but invalid hiatuses between packets (gaps between packets) occur since the selectable delay periods are discontinuous. The invalid hiatuses, which cannot be used and occur between packets, are inefficient with respect to reducing the packet discard rate and make flexible organization of traffic congestion difficult. As a means of preventing this bottleneck, optical buffer functions of a simple optical circuit structure and having a small size as well as a function enabling minute variation of the delay are demanded.
Further, in the conventional technologies, in order to obtain flexibility in terms of delay, an unrealistically large facility would be necessary. For example, if one slot time of an optical packet is 1 [μs] and the optical path length therefore is 200 [m], the transmission paths would be 09 [m], 200 [m], 400 [m], 600 [m], 800 [m], 1000 [m], 1200 [m], 1400 [m], 1600 [m], 1800 [m], 2000 [m], . . . , ∞.
Additionally, in the conventional technologies, to obtain flexibility in terms of delay, for example, the number of optical switch matrices, the number of fiber lengths downstream from optical switches, etc. would have to be increased and a large number of optical paths would be necessary. Consequently, the add loss at matrix optical switches and at m×1 and K×m couplers would increase; and SN would deteriorate as would optical transmission characteristics.
In a configuration that modulates optical packets by a single side band (SSB) modulator while looping the optical packets and that takes optical packets from the loop according to the wavelength variation consequent to the modulation, since the add loss of the SSB modulator is large (e.g., 10 [dB]), a large loss is incurred at each looping of the optical packets. As a result, the optical transmission characteristics deteriorate. Although compensation of this large add loss by an addition of an optical amplifier may be considered to prevent this occurrence, every few loops, problems arise in that optical noise generated by the optical amplifier accumulates and increases, and optical amplifiers having a large gain are costly. Furthermore, the full potential of SSB modulators is not exhibited with high speed signals, i.e., typically, at maximum, only signal light on the order of 20-25 [Gb/s] can be coped with and for example, a speed of 40 [Gb/s] is difficult to cope with.
Further, in the conventional technologies, the delay time is dependent on the number of times that the wavelength is shifted (number of loops). In addition, the disposal of an optical amplifier restricts the amplification band to approximately 30 [nm], for example. As a result, for example, wide band application, such as WDM, is difficult.